1. Field of the Invention
The present invention relates to a control apparatus for an automotive continuously variable transmission for transmitting power via a forward-reverse selector mechanism and a belt type continuously variable transmission mechanism.
2. Description of the Related Art
Conventionally, there is known as a control apparatus of this type for an automotive continuously variable transmission, a control apparatus for an automotive continuously variable transmission comprising a manual valve for changing over a forward-reverse selector mechanism, a first regulator valve for outputting a first pulley control pressure, a second regulator valve for outputting a second pulley control pressure which is lower than the first pulley control pressure, and a shift control valve for changing gear ratios for the continuously variable transmission mechanism by changeably supplying the first pulley control pressure and the second pulley control pressure to one and the other of a hydraulic cylinder of a drive pulley and a hydraulic cylinder of a driven pulley of the continuously variable transmission mechanism.
Incidentally, in general, a single pinion planetary gear mechanism is used as the forward-reverse selector mechanism, and when this occurs, the gear ratio for the reverse is set closer to the speed reducing side than the gear ratio for the forward. Due to this, an excessive torque is caused to act on the belt of the belt type continuously variable transmission mechanism when the vehicle starts to reverse, and this causes a possibility that the durability of the belt may be badly affected.
To eliminate such a drawback, conventionally JP-A-9-303550 proposes a control apparatus for controlling a continuously variable transmission mechanism such that a gear ratio thereof is changed and controlled with a shift control valve so as to be set closer to a speed increasing side at the time of reversing than forwarding.
In the control apparatus proposed in the aforesaid Japanese unexamined patent publication, a torque increase occurring due to deceleration in the forward-reverse selector mechanism at the time of reversing is offset with a torque decrease occurring due to acceleration in the continuously variable transmission mechanism to thereby prevent an excessive torque from acting on the belt of the continuously variable transmission mechanism when the vehicle starts to reverse. However, when the system is down, the shift control valve is held at the OD position where the shift control valve supplies the first pulley control pressure to the hydraulic cylinder of the drive pulley and the second pulley control pressure to the hydraulic cylinder of the driven pulley, which cannot perform the shift control of the continuously variable transmission. Due to this, the load carried by the belt when the system is down increases higher at the time of reverse running than forward running. To cope with this, there has been a demand to design belts and transmissions which can guarantee the durability thereof at the time of reverse running with the system being down, and attempting to satisfy the demand calls for increase in weight and cost.
The invention was made in view of the aforesaid drawback, and an object thereof is to provide a control apparatus for an automotive continuously variable transmission which can make the load carried by a belt of a continuously variable transmission mechanism equal between when running forward and reverse.
With a view to attaining the object, according to a first aspect of the invention, there is a provided control apparatus for an automotive continuously variable transmission for transmitting power via a forward-reverse selector mechanism in which a gear ratio for a reverse is set closer to a speed reducing side than a gear ratio for a forward and a belt type continuously variable transmission mechanism, the control apparatus comprising a manual valve for changing over the forward-reverse selector mechanism, a first regulator valve for outputting a first pulley control pressure, a second regulator valve for outputting a second pulley control pressure which is lower than the first pulley control pressure, and a shift control valve for changing gear ratios for the continuously variable transmission mechanism by changeably supplying the first pulley control pressure and the second pulley control pressure to one and the other of a hydraulic cylinder of a drive pulley and a hydraulic cylinder of a driven pulley of the continuously variable transmission mechanism, wherein when a system goes down the shift control valve is adapted to be held at such a position that the first pulley control pressure is supplied to the hydraulic cylinder of the drive pulley, whereas the second pulley control pressure is supplied to the hydraulic cylinder of the driven pulley, the control apparatus being characterized in that a control pressure changing means is provided for increasing a differential pressure between both the first and second pulley control pressures higher at the time of reversing when a hydraulic pressure is supplied from the manual valve to a reversing hydraulic pressure connecting element for changing over the forward-reverse selector mechanism to the reverse by receiving an hydraulic pressure from the manual valve than at the time of forwarding.
When the differential pressure between both the first and second pulley control pressures, even if the shift control valve is held at the OD position where the first pulley control pressure is supplied to the hydraulic cylinder of the drive pulley, whereas the second pulley control pressure is supplied to the hydraulic cylinder of the driven pulley, the gear ratio of the continuously variable transmission mechanism is shifted to the speed increasing side. Thus, according to the invention, the gear ratio of the continuously variable transmission mechanism when the system is down is changed closer to the speed increasing side at the time of reversing than forwarding so as to make the load on the belt equal between reverse running and forward running.
Here, in order to increase the differential pressure between both the first and second pulley control pressures the second pulley control pressure, which is relatively lower, may be reduced, but this produces slippage of the belt. To cope with this, it is desirable that the control pressure changing means is adapted to increase the first pulley control pressure by inputting a hydraulic pressure supplied from the manual valve to the reversing hydraulic pressure connecting element into a pressure regulating oil chamber of the first regulator valve.
In addition, in the event that the amount of increase in the differential pressure between both the first and second pulley control pressures is set such that the total gear ratio of the forward-reverse selector mechanism and the continuously variable transmission mechanism gets equal between the forward time and the reverse time, the difference in speed of the axels between the forward running and the reverse running is eliminated, and this becomes advantageous in improving the operation feeling at the time of reverse moving.